The invention relates to an adhesion-promoting composition for bonds between plastic and other materials, which is suitable for the permanent and moisture-stable bonding of, in particular, dental plastics with dental alloys.
For the optimum aesthetic fashioning of prosthetic metal structures, such as crowns and bridges, it is necessary to provide the metal surface with a tooth-coloured plastic. The plastic used in this procedure usually comprises an organic matrix based on methacrylate and dimethacrylate monomers, an initiator system, an organic filler and colour pigments. Because of the extreme conditions encountered in the oral environment, the constant changes in moisture and temperature and the high degree of mechanical stress, mechanical anchorages for the securing of the plastic are not sufficient to permanently maintain even an only nearly satisfactory bond between the dental alloy and the veneer plastic. As a rule, therefore, formation of a marginal gap of 5 to 10 xcexcm occurs between the alloy and the plastic after only a short time, the result of which can be a marked weakening of the bond to the point where the veneer is loosened. The loosening makes possible a corrosion of the metal in the region of the gap and this is accompanied by an unattractive discoloration of the veneer plastic.
Numerous suggestions have been made in recent years for the gap-free bonding of dental plastics with dental alloys. The basic principle of some of these procedures is that a silicate layer is applied to the metal surface (silicatization) in a first step and the surface is coated with a functional alkoxy silane (silanization) in a second step. The alkoxy silane, in most cases hydrolysed methacryloyloxypropyl trimethoxy silane, acts as a bonding member between the inorganic silicate layer and the dental plastic, which generally contains methacrylate. This effect is based on the fact that, on the one hand, the OH groups of the silane with the OH groups at the surface of the silicate layer are chemically bound by a condensation reaction to the silicate layer and, on the other hand, a binding to the matrix of the plastic takes place via the methacrylate group of the silane. The known procedures differ in the way in which the silicate layer is applied, while the application of the adhesion silane is virtually identical in all the procedures.
A method of applying a silicon dioxide layer to metal dental prosthesis parts by means of a high-frequency magnetron sputtering device is described in U.S. Pat. No. 4,364,731.
A procedure in which the silicate layer is applied by means of a flame hydrolysis process using tetraethoxy silane is disclosed in DE-C-34 03 894.
Moreover, a procedure in which a silicate/chromium oxide layer is applied to the surface of a dental alloy by a sol/gel solution and solidified by a subsequent heat treating process at 320xc2x0 C. for 2 to 8 minutes is disclosed in DD-A-276 453.
A procedure in which the silicate coating takes place by means of a corundum jet process, adding a certain quantity of silicon dioxide having an average particle size of  less than 5 xcexcm to the sprayed corundum, is described in DE-A-38 02 043. Local energy densities, which are sufficient to melt the finely dispersed silicate particles onto the metal surface, occur in the area in which the corundum particles impact.
Finally, a procedure in which, instead of the inorganic silicate layer, an organic phenolic resin/acrylate/methacrylate layer serves as the bond layer, is described in DE-A-42 28 530.5.
With the described bond procedures, in particular with the sputtering process, the flame hydrolysis procedure and the sol/gel procedure, it is necessary, for the bond layer to adhere strongly to the surface of the alloy, to heat the latter to a temperature of about 300xc2x0 C. However, such a high temperature proves disadvantageous with certain alloys, e.g. alloys with a copper content of  greater than 5%, as it leads to a change in the composition of the alloy at the surface. This also results in a reduction in adhesion and discoloration of the alloy. Moreover, the known procedures require a major outlay on apparatus, or the use of expensive chemicals, if they are to be carried out.
The object of the invention is therefore to provide an adhesion-promoting composition for bonds between plastic and other materials, which can be used with only a small outlay on apparatus and exposing the material to a small temperature stress, and which makes it possible to produce a strongly adhesive and moisture-stable bond layer and through the latter a permanent, high-strength connection, free of marginal gaps, between plastic and other materials.
This object is achieved by the adhesion-promoting composition according to claims 1 to 7. The invention also relates to the use of the composition according to claims 8 to 9, to a process for the production of bonds between plastic and other materials according to claims 10 to 15, and to the material according to claim 16 having a bond layer.
The adhesion-promoting composition according to the invention for bonds between plastic and other materials is characterized by the fact that it contains (a) at least one alcoholate of titanium, of zirconium or of hafnium.
The alcoholates used are from time to time also described as esters of titanium, zirconium and hafnium.
The alcohol moiety of the alcoholates used according to the invention can be derived from aliphatic or aromatic alcohols.
Alcoholates derived from aliphatic C1 to C5 alcohols, such as in particular tetrabutyl titanate, tetrapropyl titanate, tetraisopropyl titanate or zirconium(IV) propylate, are preferred.
It is preferred that the composition also contains (b) at least one alkoxy silane having at least one further functional group. Coming into consideration as a further functional group are in particular a vinyl, methacryl, acryl, glycidyl or amino group, with combinations of different functional groups being also possible. The alkoxy silanes generally contain 1 to 3 alkoxy groups, with methoxy and ethoxy being preferred. Special examples of particularly suitable alkoxy silanes are vinyl-trimethoxy silane, xcex3-methacryloxypropyl trimethoxy silane, 3-glycidoxypropyl triethoxy silane and aminoethylaminopropyl trimethoxy silane.
Finally, it has also proved suitable that the composition also contains (c) a solvent for the alcoholate (a) and, if present, for the functional alkoxy silane (b). In view of the high reactivity of the alcoholates with water, anhydrous solvents are preferred. Acetone, acetic acid ethyl ester, isopropanol or hexane come into consideration in particular.
If a combination of alcoholate (a) and functional alkoxy silane (b) is used, then these can be applied jointly, e.g. in the form of a single-component solution, or successively, i.e. in the form of two solutions. The alcoholate and the alkoxy silane can be present together in the form of a packaging unit in which both components are present separate from each other. The two components can be released from such a unit immediately before use and e.g. mixed to give a single-component solution. Particularly preferred solutions have the following compositions:
Solution 1: 5 to 50 ml of a titanium alcoholate or zirconium alcoholate
100 ml solvent
Solution 2: 5 to 10 ml functional alkoxy silane
100 ml solvent.
By mixing solutions 1 and 2, a solution 3 can be produced which can be used as a single-component solution.
Solution 3: 5 to 50 ml titanium alcoholate or zirconium alcoholate
5 to 10 ml functional alkoxy silane
200 ml solvent.
It has been shown that the composition according to the invention is outstandingly suitable for improving the bond between plastic and other materials, such as in particular metals, including alloys, ceramics, glass ceramics or glasses.
For this reason it is also preferably used as a dental material. With its help it is possible to provide dental alloys of very different compositions with a bond layer which, through its adhesion-promoting action, leads to a very good binding of the dental alloy to dental plastics. In addition to the production of e.g. plastic/metal bonds, metal can also be bonded to metal, with both metal materials being provided with the composition and joined to each other by using a dental adhesive. The bonds obtained show by high strength even after frequent exposure to temperature change and prolonged storage in water at elevated temperatures.
The composition according to the invention can also be preferably used as an adhesion promoter for paint coatings in mechanical engineering and body construction.
The process when producing bonds between plastic and other materials is that
(i) the composition according to the invention is applied to the material and the alcoholate is hydrolysed and
(ii) the plastic is applied to the material that has been provided with the composition.
If, as well as the alcoholate, alkoxy silane is present in the composition, then these two components can be applied either jointly, e.g. in the form of a single-component solution, or successively, e.g. in the form of separate solutions. In the latter case, the alcoholate is preferably applied first, and the alkoxy silane only afterwards.
Following the application of the composition in step (i) to the material, hydrolysis takes place and thus also condensation of the highly reactive titanium, zirconium or hafnium alcoholates by the OH groups present at the surface of the material or the water bound by chemisorption at the surface. Materials exposed to normal humidity regularly contain a proportion of surface OH groups or of water bound by chemisorption which is sufficient for the hydrolysis of the alcoholates to take place to an adequate extent. Finally, as a result of the hydrolysis and condensation, a TiO2, ZrO2 or HfO2-containing layer forms which adheres strongly to the surface of the material.
In order to complete this reaction and remove the water and alcohol that have formed as a result of the reaction and any solvent that may be present, a heat treatment can be carried out, at a temperature of in particular less than 300xc2x0 C., and preferably at temperatures between 100 and 200xc2x0 C., after the composition has been applied. The temperature treatment is usually carried out only after any solvent present has substantially evaporated.
Through the simultaneous or optionally consecutive use of the functional alkoxy silane (b), silanization of the formed TiO2, ZrO2 or HfO2-containing layer takes place. The functionality of the alkoxy silane makes possible the binding to the matrix of the plastic applied in stage (ii). It is possible to match the functionality of the alkoxy silane to the plastic selected in each case. Particularly preferred plastics are plastics used in the dental field and in particular plastics based on acrylates or methacrylates. The plastic is usually applied by application of a corresponding monomer mixture and its curing.
Compared with the state of the art, the following advantages are obtained with the composition according to the invention and the process according to the invention:
the applied inorganic bond layer prevents a water diffusion, weakening the bond, to the surface of the material,
the solidification of the bond layer can take place at room temperature or through temperature treatment at temperatures of up to 200xc2x0 C., with the result that there is no change weakening the bond, e.g. at alloy surfaces,
the bond process according to the invention can be used regardless of the composition of alloys,
the process according to the invention can be realized for only a small outlay on apparatus such as simple heat sources.
Finally, the invention also relates to a material based on metal, ceramic, glass ceramic or glass, which has a bond layer which can be obtained by the application, described above, of the compositions according to the invention to the surface of the material and hydrolysis of the alcoholate (a). Such materials are prepared for a later secure connection with plastic in an excellent manner.
Other advantageous embodiments of the invention and also in particular its use in dentistry are explained in the following examples.